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Re: Lunar trouble, need help
From: Kent Nordström
Date: 2008 Jun 16, 11:31 +0200
From: Kent Nordström
Date: 2008 Jun 16, 11:31 +0200
Sorry for misprinting. The sun's apparent time shall be 15-59-47.5. The
calculation of MT-GMT is correct.
Kent N
----- Original Message -----From: Anabasis75@aol.comTo: NavList@fer3.comSent: Monday, June 16, 2008 5:04 AMSubject: [NavList 5472] Re: Lunar trouble, need helpThanks Frank.That is perhaps what I did. I can't be sure at this point. Unfortunately, since I don't have reduction tables or programs at sea, I can't clear my sights until I return to port, and by that time, I have forgotten some of the fine details. I will try and remember to do an IC check on the sextant tonight, it might be the source of some error since I haven't checked in awhile.On a slightly different subject, what is your sign convention on your lunar program? When I get a positive error in longitude, is that east or west of the DR position? I am trying to deterimine the position north or Hawaii and am running into trouble with sign convention.JeremyIn a message dated 6/16/2008 8:33:48 A.M. West Pacific Standard Time, frankreed@HistoricalAtlas.net writes:
Hi Jeremy.
I have a little working theory on your lunars. I suspect that you took what
I call "Inside Limb" lunar distances rather than "Near Limb" lunars. With
Near Limb lunars, you bring the Moon over to the Sun so that they just touch
with the two disks side-by-side with no overlap and no gap between them.
With an 'Inside Limb' shot you bring the Moon's disk inside the Sun's disk
and bring it into contact with the far limb (note that this is not what is
meant by a "Far Limb" lunar in which you bring a star at night over to the
limb opposite the star). Assuming that this is the case, we can convert your
Inside shot to a Near shot by subtracting twice the Sun's semi-diameter
which would be 31.5'. Then the average error in the set is 0.8 minutes of
arc (which may indeed be due to an error in index correction as Jim
suggested) and the standard deviation of that set is around 0.25'. Shooting
an "Inside" lunar is not normal practice, but if you've never seen lunars
done or diagramed, there's no way that you would know that. I think it's
harder to shoot those alignments than a standard Near Limb lunar.
Of course, this is just a speculation. If you're quite sure you shot these
lunars by bringing the Moon and Sun side-by-side, then the problem must lie
elsewhere.
-FER
PS: If you select "Far Limb" lunar in my online calculator for a Sun sight,
you get a result that really doesn't mean anything since Far lunars actually
don't make any sense when the other body is the Sun. Maybe I will change
that to calculate Inside Limb lunars.
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